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8/2/2019 Lecture 1 Water (1) Printed
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“We acknowledge
the Traditional
Owners of this
Land, the Nyungar
people, and pay
respect to the Elders
of their community.”
Water in Food
Water in Food Systems• Solvent - Salts, sugars, starch & proteins
• Dispersion medium - Colloids
• Reaction medium - Hydrolysis
• Plasticizer & lubricant
• Stabilizer of macromolecule conformation - Formation of H bonds
• Cost - Dehydration / transportation / wilting
• Other – Spoilage and microbial growth
Structure of Water
• Oxygen has 6 and hydrogen has one electrons in outer
shell
• Hydrogen and oxygen share an electron pair, forming a
covalent bond• These 4 electron pairs try to arrange themselves as far
away from each other as possible
10405’
8/2/2019 Lecture 1 Water (1) Printed
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Structure of Water
• The positive and negative charges in water
are not evenly distributed
• Molecule is partial positive and partialnegative (water is a POLAR molecule)
• Participates readily in hydrogen bonding
Hydrogen Bonding
• Weak bonding• Can occur in polar
molecules such as water
• In water, each molecule is
attached to 3.5 others
molecules
• In ice, attached to 4 other molecules
Principles of food chemistry
DeMan, 1999
Water • Unique
– Only compound abundant in three physical states
– Solid/liquid/gas
• Essential for life – Metabolic processes
• Major component in foods
• Influences on the structure, appearance and taste of foods
– Dried vs. fresh
• Influence the susceptibility to spoilage
Moisture Contents of Various Foods
FoodWater Content (%)
Pork (raw) 53-60Beef (raw) 50-70
Berries 80-85
Apples 90
Strawberries 90-95
Avocados 74-80
Potatoes 85-90Lettuce 90-95
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Physical Properties
• Compared to other similar molecules,water has very high values for
– Melting point
– Boiling point
– Surface tension
– Dielectric constant [permittivity]
– Heat capacity
Physical Properties• Compared to other similar molecules, water has
very high values for
– Heats of phase change
• The heat of fusion – Amount of energy given off when a
substance freezes
– H fusion 80 cal/gram = 1.44 kcal/mole
• The heat of vaporization
– Amount of energy given off when a gasliquefies
–H vaporization = 560 cal/g = 10.5kcal/mole expect 8-16 Kcal/mole
• A lot of these anomalies are due to the extent of hydrogen bonding
Molecular Weight
BoilingPoint °C
SurfaceTension
Water (H2O) 18 100 73Hydrogen Sulphide(H2S)
34 -60 NA
Ammonia (NH3) 17 -33 NAMethanol (CH3OH) 32 65 22Ethanol (C2H5OH) 46 78 22Ether (C2H5OC2H5) 74 34 17
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Boiling Points of Chemical Compounds Physical Properties
• Moderately low density
– Less dense as solid than as liquid
• Normal viscosity
• Result of strong intermolecular forces in water
Molecular Structure Molecular Structure - Ice• Ice has a well defined structure
– Each water molecule is H-bonded to 4surrounding water molecules
– Two are H-bonded to the central oxygen; Thetwo H atoms are bonded to neighbouring Oatoms on neighbouring H2Os
Water Ice
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Molecular Structure - Water
• Partial retention of hydrogen-bonded tetrahedral
structure of ice.
– Thermodynamically favoured
• Rapid breaking and reforming of bonds to allow
movement
Molecular Motion
• Vibrational
– Around own bonds
– Ice, water and steam
• Rotational
– Spin on the spot
– Water and steam
• Translational – Move between regions
– Steam
Melting
Freezing
Sublimation
Deposition
Condensation
Vaporization
Triplepoint of
water =0.01 C and0.006 atm
Chemical Bonding
• Several
common bond
types inmolecules
– Hydrogen
– Hydrophobic
– Ionic
– Covalent
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Bonding - Ionic
– Interacts strongly with charged ions
– Increases solubility of polar/chargedsubstances
• Eliminate ionic attraction
– Ions (Na+, Cl-)
• Alter the structure and mobility of water
©1999 by Stephen K. Lower; all rights reserved. November 10, 1999
Chem1 General Chemistry Reference Text States of matter
Bonding - Ionic
Bonding - Hydrogen
• Water competes for thehydrogen bonding sites
• Important in proteins – H bonds between side
chains and amide groupsstabilise structure
Interactions - Hydrophobic
• Hydrophobic groups
prefer a lipophillic or
non-aqueous
environment.
– Move away from
water, toward
each other
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Water in Foods
• “Water Binding” and “hydration” refer to the
ability of water to associate with hydrophilicsubstances in food.
• Even though this water may be associated or
even trapped with the food the individual water
molecules may be free or not associated.
Water in Foods
• The interactions of individual water molecules
with various solutes produces what is termed“bound water”.
• The amount of water present in a food that is in
the “bound” form or associated with bound water
is also important in determining the shelf life of
the food.
Forms of Water in Foods
1. Bound
2. Capillary or monolayer
– Monolayer surrounding macromolecules
3. Free water
– Bulk water
Forms of Water - Bound• Strongly bound
– Constitutional water
• Not available as a solvent or to support enzymeactivities – Molecular level
• Un-freezable water
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Forms of Water - Capillary
– Successive layers over the macromolecule
– Mostly not freezable at - 40C
– Influences on the physical properties of
solutes
– Increases the rate of most reactions
Forms of Water – Free Water
– Fills capillaries and pores
– Retains all the properties of “water”
• Increases molecular mobility
• Decreases viscosity
– Freezable
– Physically constrained within the
macromolecular matrix
Freezing of Water
• Freezing itself is not lethal to micro-organisms
• Increase the solute concentration
– Osmotic pressure
– pH
• Freezing is not an effective means of destroying
micro-organisms
Moisture Content
• Wet basis or ‘as is’
Weight of H2O / Weight of sample
• Dry basis
Weight of H2O/ Weight of sample (dry)
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Water Activity
• Generally foods with a greater moisture content
are more perishable
• But some foods with the same water content
differ significantly in perishability
– Due to the difference in extent that water
associates with nonaqueous constituents
– Intermediate moisture foods
Water Activity
• Water activity is defined as the ratio of the
vapour pressures of pure water and a solution at
a given temperature:
aw =
o
where aw - water activity
- partial pressure of water in foodo - vapour pressure of water at the
same temperature
The Concept of Water Activity
• The availability of water for
– Physical
– Chemical and – Biochemical reactions
• Physical properties of biological materials aremodified by the presence, amount and activity of water
• Critical factor in the stability of foods
Water Activity
• NOT the total moisture content which includes“bound water”
• It is a measure of the water available to microbesfor growth.
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Determination of Water Activity
• %ERH is the measured variable.
Range of Water Activity Values
• 0 (bone dry) - 1.00 (pure water)
• expressed as a decimal fraction of the
equilibrium relative humidity (ERH) of a
particular material.
• A sample containing a water activity reading of
0.91 is equivalent to an (ERH) reading of 91%.
Water activity is not directlyrelated to the moisture
content?
Critical Aw
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Bound and Free Water Temperature Dependence of Water Activity
Aw of Common Foods
• >0.95 Fresh fruit, meat, milk
• 0.95-9 Cheese
• 0.9-0.85 Margarine,• 0.85-0.8 Salted meats
• 0.8-0.75 Jam
• 0.75-0.65 Nuts
• 0.65-0.60 Honey
• 0.5 Pasta
• 0.3 Cookies
• 0.2 Dried vegetables, crackers
• Gram –ve 0.97
• Gram +ve 0.90
• Yeasts 0.88
• Moulds 0.80
• Halophiles 0.75
• Xerophiles 0.65
Aw and Changes in Foods
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Aw and Changes in Foods
• Mobility of components depend on the available
water
• Bring components together
• Dilution may be a factor – at high Aw levels or
being a product of the reaction
Physical Preservation and Aw
• Preservation without the addition of antimicrobial preservatives
• Utilised in a wide variety of food preservationprocedures
– Evaporation
– Drying
– Salting
– Brining
– Freezing
• Reduce the amount of free water
Physical Preservation and Aw
• Absence of chemical preservatives,
– Aw determines the ability of a system to resistmicrobial growth.
– More tightly water is bound, the lower itschemical potential and Aw.
• Ideal ingredient would have a high moisturecontent with a low water activity.
• Low Aw does not prevent the presence of
microorganisms, it only limit the micro-organismgrowth.
Control of Water Activity
• Include materials with lower water activity
– Salt - Calcium and sodium
– Glycols (PEG’s, propylene glycol) – Alcohols
– Sugars (sorbitol, high fructose corn syrup)
– Glycerin
– Gums (xanthan)
– Proteins
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Control of Water Activity
• Substances bind water
– Hydroxyl groups of sugars
– Carbonyl and amino groups of proteins
– Other polar sites that hold water by hydrogen
bonding
– Dipole-dipole forces
– Van der Waals forces
– Ionic bonds
Processing
S e n s o r y q u a l i t y / s h e l f - l i f e
Formula / Aw
Fq Fs
Shelf life
Water activity
Fs = Formula that gives the required shelf-life
Fq = Formula that gives the required quality
The Moisture Sorption Isotherm• The relationship between the equilibrium moisture
content of a material (g of water/g of dry matter) and
water activity, at a given temperature.
• Measured by addition of water to a dry sample
– absorption
• Or by removal of water from a wet sample
– desorption
Region A / I
Region B /II
Region C / III
Aw of a product
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The Moisture Sorption Isotherm
• The differences ‘hysteresis’
• Hysteresis could be due to change in theavailability of active polar sites after drying
• During drying
– The polar sites draw more closely together tosatisfy each other
– Reduces the water holding of the material onsubsequent hydration.
The Moisture Sorption Isotherms
Significance
• For concentration and dehydration
processes• To avoid moisture transfer among the
ingredients
• To determine the moisture barrier properties needed in a packaging material
• To determine what moisture content willcurtail growth of microorganisms of
interest• To predict the chemical and physical
stability of food as a function of water content
Hydroscopic?
Composition Effects
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Composition Effects – 20% fatSorption Isotherm for Mixtures